This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2004-208928 and Patent Application No. 2004-258684 filed in Japan respectively on Jul. 15, 2004 and Sep. 6, 2004, the entire contents of which are hereby incorporated by reference.
The present invention relates to induction heating devices and image forming apparatuses equipped with such induction heating devices that can be suitably used with such devices as fixing devices in dry process electrophotographic devices, drying devices in wet process electrophotographic devices, drying devices in inkjet printers, and erasing devices for rewritable media.
In heating devices, for example the fixing devices in dry process electrophotographic devices, the drying devices in wet process electrophotographic devices, and the heating devices used in erasing devices for rewritable media, a structure that has been long and widely used is one in which a halogen lamp is provided inside a heating roller made of aluminum or the like having a hollow core, and in which the heating roller is subjected to radiation heating by the halogen lamp. Since systems using these halogen lamps are indirect heating systems, there are the problems that the startup time at the commencement of heating is slow and the warm up time is long.
Accordingly, attention is being given to induction heatng devices in which a conductive layer is provided on the heating roller, and an eddy current is produced in the conductive layer by applying alternating magnetic fields produced by a magnetic field producing means, such that a heat generating layer is directly heated by the Joule heat of the eddy current. These induction heating devices directly heat the structure to be heated, and therefore have superior heating efficiency, but in order to achieve shorter warm up times and to further improve practical convenience, it is necessary to achieve low thermal capacity in the heating roller.
One technique for achieving low thermal capacity in the medium to be heated in induction heating devices is to make the heating roller into a belt form, but in this case new problems are presented such as a mechanism being required to prevent meandering of the belt. A technique for achieving low thermal capacity in a roller-form structure to be heated has been proposed as a method for solving this (see JP 2002-49261A for example [hereinafter referred to as patent document 1]).
While setting the thickness of the heat generating layer thinner than the thickness of the surface layer achieves lower thermal capacity, there is insufficient mechanical strength, and therefore the method described in patent document 1 achieves mechanical strength as well by using a structure in which the heat generating layer is backed up by a low thermal conduction layer and a high thermal conductive cylindrical rigid body.
Furthermore, in order to further reduce the heating time in conventional belt systems using halogen lamps as the heat source, a heating device has been proposed that also incorporates an induction heating means that heats the belt portion (see JP 2003-228249A for example [hereinafter referred to as patent document 2]).
Further still, although for a different purpose, a method in which heat sources are provided inside and outside the heating roller has been proposed (see JP 2001-343860A for example [hereinafter referred to as patent document 3]). The art described in patent document 3 has a different purpose than the art of the two above-mentioned patent documents 1 and 2. For example, depending on the image to be copied, there are areas in which no image information is present, and heating these areas results in consuming energy for no purpose. For this reason, the art of patent document 3 involves adjusting the electrical power of the outside heat source according to the image pattern, and an induction heating system is proposed as one of those outside heating means.
In this regard, with the above-mentioned patent document 1, the time taken to raise the temperature of the heat generating layer is reduced and the warm up time is reduced by setting the heat generating layer thinner than the surface layer to lower the thermal capacity and by arranging a low thermal conductive member between the heat generating layer and the cylindrical rigid body to prevent heat loss from the heat generating layer.
However, since the low thermal conduction layer has a strong thermal blocking effect, the temperature of the low thermal conduction layer becomes lower than the heat generating layer at the point when the surface of the heating roller reaches the predetermined temperature, that is, at the point in time immediately after warming up. In other words, since only the heat generating layer portion that has been made to have low thermal capacity is made to store heat at a high temperature, when continuous copying is carried out under this condition, heat other than the heat conveyed to the unfixed image escapes from the heat generating layer to the low thermal conduction member, and therefore the surface temperature of the roller cannot be maintained and there is the issue of fixing nonconformities occurring due to temperature drops.
Furthermore, since induction heating systems are localized heating systems in which generated heat is greater concentratedly in the heat generating layer that is closer to the heating coil, large temperature unevenness is produced in the circumferential direction of the heating roller when heating is carried out while the heating roller is in a stationary state. For example, FIG. 11 is a drawing for describing the transition in roller surface temperature when preheating is carried out in a stationary condition and the roller is made to return to normal after this condition. As is evident from FIG. 11, there are temperature peaks in the areas being preheated by the heating coil, and therefore it is understood that there are high temperature places and low temperature places in the circumferential direction of the roller. This temperature unevenness in the circumferential direction is particularly large in the case of external heating systems with temperature unevenness of 30° C. or more being produced.
The only method for solving this is a method in which temperature adjustment is carried out over a long time while rotating the heating roller to achieve uniformity, and thus there is an issue in that it is difficult to reduce the time for the first copy. That is, circumferential direction unevenness is solved when preheating while rotating, but there is a separate issue in that the amount of heat released increases, and therefore the amount of electrical power consumed during preheating increases and energy efficiency cannot be achieved.
With the art described in patent document 2, the heating of the belt is carried out using thermal contact conductance from a heating roller (in which a halogen lamp is arranged inside) installed in a position apart from the fixing nip portion and by induction heating by a heating coil installed between the heating roller and the fixing roller, and therefore there are many opportunities for heating the belt and it is possible to raise the temperature of the belt in a short time. However, since there is no heat source at the fixing roller portion that forms the fixing nip portion for fixing the toner, there is an issue in that when the temperature of the fixing roller is low, the temperature of the fixing belt drops during continuous copying, thus causing fixing nonconformities. Furthermore, temperature unevenness in the circumferential direction of the belt is produced since the positions of the fixing nip portion and the heating sources are different, and therefore there is an issue in that it is difficult to reduce the time of the first copy, which is the same for the above-mentioned patent document 1.
On the other hand, the art described in patent document 3 was devised giving consideration to the point that it is inefficient to uniformly heat areas in which toner (image) of the unfixed image is present and areas in which it is not present. A specific characteristic here is that only areas in which toner (image) is present are selectively heated by the external heating means. Furthermore, a main characteristic is that the surface temperature of the roller, which has dropped due to passing through the fixing nip, can be made to return to the predetermined temperature by regulating the electrical power of the external heating means when in conditions in which it is difficult to maintain the roller temperature such as in conditions of continuous copying. Consequently, the art of patent document 3 is not for controlling the thermal storage temperature conditions of the elastic support layer that supports the heat generating layer, and has issues the same as in the art of the above-mentioned two patent documents in that only the surface heat generating layer tends to a high temperature state and the surface temperature of the heating roller tends to drop easily.